High-speed imaging devices (high-speed video cameras) for taking consecutive images of high-speed phenomena, such as explosions, destructions, combustions, collisions or discharges, for only a short period of time have been conventionally developed (for example, refer to Non-Patent Document 1 and other documents). Such high-speed imaging devices need to perform an ultrahigh-speed imaging operation that exceeds a level of approximately one million frames per second. Accordingly, they use solid-state image sensors capable of high-speed operations, which have special structures different from those of the imaging sensors conventionally used in normal video cameras, digital cameras and similar devices.
One example of this type of solid-state image sensor is disclosed in Patent Document 1 and other documents. The devices disclosed in those documents are referred to as an in-situ storage image sensor (IS-CCD). An outline of this image sensor is as follows.
In this in-situ storage image sensor, a storage CCD for a specified number of record frames is provided for each photodiode functioning as a photo-receiver. This CCD also serves for transferring signals. During an imaging operation, pixel signals produced by photoelectric conversion by the photodiode are sequentially transferred to the storage CCD. After the imaging operation is completed, the pixel signals corresponding to the specified number of record frames stored in the storage CCD are collectively and sequentially read, and the images corresponding to the specified number of record frames are reproduced outside the image sensor. During the imaging operation, pixel signals exceeding the specified number of image frames are discarded from the oldest ones. Thus, the latest set of pixel signals corresponding to the specified number of frames are always held in the storage CCD. This means that, when the transfer of pixel signals to the storage CCD is suspended at the completion of the imaging operation, one can obtain the latest series of images ranging from the completion of the imaging operation back through a period of time corresponding to the specified number of record frames.
Thus, unlike general types of image sensors that require pixel signals to be extracted every time a set of pixel signals corresponding to one frame is obtained, the in-situ storage image sensor is characterized by its capability of acquiring a plurality of consecutive images at extremely high speeds. However, the in-situ storage image sensor has the following problems.
(1) The aforementioned in-situ storage image sensor structurally requires a large amount of power. This is because the gate electrodes and signal lines, both having high capacitive loads, must be simultaneously driven during the process of transferring signal charges to the CCD, and also because the voltage amplitude of the gate-driving signal is large. Attempting to increase the drive speed for higher imaging speeds would additionally increase the power consumption and possibly cause heat generation that exceeds the upper limit of heat radiation. Furthermore, the high capacitive load may cause waveform distortion of the drive signal (e.g. dulling of the waveform), in which case the attempt of increasing the drive speed may totally prevent the charge transfer.
(2) In the conventional in-situ storage image sensor, the storage CCDs are located between the neighboring photodiodes on a semiconductor chip. Therefore, when a large amount of photocharges are generated by an incidence of strong light onto a photodiode, some of the photocharges may flow into the storage CCD. As a result, various problems can occur, such as the deterioration of signal-to-noise (S/N) ratio of the images. In the worst case scenario, the image will be ghosted.
(3) Even during the stand-by period for the next signal-reading cycle, the storage CCDs receive false signals due to dark charges. This may possibly lower the S/N ratio of the images.
On the other hand, in the field of CMOS image sensors, a device disclosed in Patent Document 2 has been known. In this image sensor, a plurality of capacitors acting as memory elements are provided within each pixel so that the photocharges generated by the photodiode can be consecutively stored, for each frame, in a different set of memory capacitors. Although this device can continuously record a few to several frames, it cannot perform the continuous imaging operation over such a large number of frames that the aforementioned high-speed imaging device is intended for. Using more memory capacitors to increase the number of continuously recordable frames results in a higher capacitive load of the signal lines extending from the detection node storing the charges generated by the photodiode, which makes it difficult to raise the operation speed. Furthermore, the area of the photodiode within each pixel inevitably decreases, which lowers the aperture ratio and accordingly deteriorates the sensitivity.